Many different means of security are available to prevent the duplication of printed commercial documents such as special papers (water marked paper or metal fiber filled paper) and special inks (fluorescent ink and other optically variable inks) which form latent images that change color under U.V. light or repel water. Examples of special papers and inks are described in U.S. Pat. Nos. 4,150,997, 4,153,593, 4,328,332 and 5,468,581. While the use of special papers and special inks as security measures has been effective and versatile, with the advent of today's personal computers and color copiers, conventional security features such as these have been overcome, particularly where fluorescent images and water marks are used in environments where records are only casually inspected, such as sales receipts and transaction records. Therefore, it is desirable to provide additional security measures which either replace or supplement conventional security features.
Providing security measures to paper is complicated by many factors. One is that the many types of printing inks and papers are often adapted to be employed in particular printing operations. For example, the papers used in ink jet printing require special coatings to provide high resolution and the inks used in ink jet printing often must be conductive, have viscosity values within a certain range and contain no large particulate matter. Security features cannot interfere with these requirements. Similarly, the papers and inks employed in thermal transfer printing, relief printing, offset printing, intaglio printing, flexographic printing, thermal printing, lithography and silk screening have features which must be considered when a security feature is provided.
Where the paper or ink formulation already has a security measure, adding a security feature to a security paper is more difficult in that the new security feature must complement the performance of the existing features without conflict. Another factor to consider is that it is often desirable to provide security features "on-demand" where special papers are used for commercial documents. There are also practical limitations on security features in that they must be easy to apply and use so as not to add significantly to the cost of the transaction. Parameters such as these restrict the availability of security features, making it difficult to employ new or multiple security measures for a transaction receipt which cannot be easily circumvented.